The Hippopotamus (Hippopotamus amphibius also known as river-horse) is a large, plant-eating African mammal, one of only two extant, and three or four recently extinct, species in the family Hippopotamidae.

Size: Hippos average 3.5 meters (11 ft) long, 1.5 meters (5 ft) tall at the shoulder, and weigh from 1,500 kg to 3,200 kg (3,300 to 7,000 lb). They are approximately the same size as the White Rhinoceros, and experts are split on which is the next largest land animal after the elephant. Male hippos appear to continue growing throughout their lives, whereas the females reach a maximum weight at around the age of 25. Females are smaller than their male counterparts, and normally weigh no more than 1,500 kg. The value given above of 3,200 kg is often quoted as being the upper limit of weight for a male hippo. However, larger specimens than this have been documented, including one of which weighed almost 5,000 kg.

Speed: Even though they are a bulky animal, hippopotamuses can run faster than a human on land. There are estimates of its actual running speed varying from 30 km/h (18 mph) to 40 km/h (25 mph), or even 48 km/h (30 mph). The hippo can maintain these higher estimates for only a few hundred yards.

Head: The eyes, ears, and nostrils of the hippo are placed high on the roof of the skull. This allows them to spend most of the day with the majority of their body submerged in the waters of tropical rivers to stay cool and prevent sunburn.

Skin: For additional protection from the sun, their skin secretes a natural sunscreen substance which is red colored. This secretion is sometimes referred to as "blood sweat," but it is not actually blood, nor sweat. This secretion starts out colorless, turns red-orange within minutes, eventually becoming brown.

Relationship to other animals: As indicated by the name, ancient Greeks considered the hippopotamus to be related to the horse. Until 1985, naturalists grouped hippos with pigs, based on molar patterns. However evidence, first from blood proteins, then from molecular systematics, and more recently from the fossil record, show that their closest living relatives are cetaceans – whales, porpoises and the like. Hippopotami have more in common with whales than they do with other artiodactyls (even-toed ungulates), such as pigs. Thus, the common ancestor of hippos and whales existed after the branch-off from ruminants, which occurred after the divergence from the rest of the even-toed ungulates, including pigs. While the whale and hippo are each other's closest living relatives, their lineages split very soon after their divergence from the rest of the even-toed ungulates.

Combat: Hippos are highly territorial; a male hippo often marks his territory along a riverbank from which to draw in a harem of females, while defending it against other males. Male hippos challenge one another with threatening gapes. Their canine teeth are 50 cm (20 inches) long, and it uses its head as a battering ram, especially against rival males while fighting over territory.

Danger to man: Since their habitat is often encroached upon by farmers and tourists, and because they are so territorial, the hippopotamus is one of the most dangerous animals in Africa. They are said to account for more human deaths than any other African mammal. The hippo does not hunt humans, but defends its own territory vigorously.

Water Habits: Hippos are usually found in shallow water, and rarely come out of that depth. Most hippos that look as though they are floating are in fact standing or lying on the bottom.

Diet: They feed on land mostly at night, consuming as much as 50 kg (110 lb) of vegetation per day. They have been known to occasionally scavenge meat from animals found near their range, but hippos are not carnivorous in any real sense.

Movement in water: Adult hippos are not generally buoyant. When in deep water, they usually propel themselves by leaps, pushing off from the bottom. They have been observed to move at 8 km/h in water. Young hippos are buoyant and more often move by swimming, propelling themselves with kicks of their back legs. One hippo calf survived after being pushed out to sea during the tsunami generated by the 2004 Indian Ocean earthquake and was rescued on a nearby island.

Babies: Baby hippos are born underwater at a weight between 60 to 110 pounds and must swim to the surface in order to take their first breath. The young often rest on their mothers' backs when in water that is too deep for them, and swim underwater in order to suckle.

Breathing: Adult hippos typically resurface to breathe every 3-5 minutes. The young have to breathe every 2-3 minutes. The process of surfacing and breathing is automatic, and even a hippo sleeping underwater will rise and breathe without waking. Hippos have been documented staying submerged for up to 30 minutes. A hippo closes its nostrils when it submerges.

Hippopotamuses, also called hippos, are gregarious, living in groups of up to 40 animals, called a pod, herd, school or bloat. A male hippo is known as a bull, a female, a cow, and a baby, a calf. A hippo's lifespan is typically 40 to 50 years. Female hippos will reach sexual maturity at 5 to 6 years, and have a gestation period of 8 months.

Extinction: Three species of hippos became extinct within the Holocene on Madagascar, one of them as recently as about a thousand years ago. A dwarf species, Phanourios minutis, existed on the island of Cyprus but became extinct at the end of the Pleistocene. Whether this was caused by human intervention is debated (see Aetokremnos). In 2005, the population of hippos in Democratic Republic of the Congo's Virunga National Park had dropped to 800 or 900 individuals from around 29,000 in the mid 1970s, raising concerns about the viability of that population. This decline is attributed to the disruptions caused by the Second Congo War. The poachers are believed to be former Hutu rebels, poorly paid Congolese soldiers and local militia groups.

The poachers hunt due to hunger but also for money. A three-tonne hippo is worth thousands of dollars. The sale of hippo meat is illegal, but when the meat arrives unannounced in markets, it sells so fast that it is difficult for the WWF officers to track. Conservationists warn that the hippo may soon be extinct in Congo.

Conservation status: Five subspecies of hippos have been described based on morphological differences (H.a. amphibius, H.a.kiboko, H.a.capensis, H.a.tschadensis, H.a.constrictus; Lydekker 1915).

However, the existence of these putative subspecies had not been tested by genetic analyses. A recent paper by Okello et al. (2005) does just that. Using mitochondrial DNA from skin biopsies taken from 13 sampling locations, the authors consider genetic diversity and structure among hippo populations across the continent. They find low but significant genetic differentiation among 3 of the 5 putative groups - H.a. amphibius, H.a.capensis, H.a.kiboko. If these findings are accurate, that would mean that common hippos in Kenya and Somalia (kiboko), southern Africa (capensis from Zambia to South Africa), and the rest of sub-Saharan African countries (amphibius) represent three distinct subspecies, with H.a.amphibus as the ancestral group.

Okello et al. also find evidence that common hippos in Africa experienced a marked population expansion during or after the Pleistocene Epoch, which they attribute to an increase in water bodies at the end of this era. These findings have important conservation implications. Hippo populations across the continent are threatened by habitat loss and unregulated hunting. In addition to addressing these common threats, the genetic diversity of these three distinct subspecies will need to be preserved. The hippopotamus has been moved on to the so-called "Red List" drawn up by The World Conservation Union (IUCN) in May 2006. This signifies that the common hippopotamus is now in serious danger of extinction.

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